Allylpalladium Complexes with <i>P</i>-Stereogenic Monodentate Phosphines. Application in the Asymmetric Hydrovinylation of Styrene

A group of P-stereogenic monodentate phosphines S-PPhRR‘ (R = 1-naphthyl, 9-phenanthryl, or o-biphenylyl and R‘ = CH3−, i-C3H8−, and Ph3SiCH2−) have been prepared by succesive substitution reactions on the oxazaphospholidineborane obtained from (−)-ephedrine and bis(N,N-diethylamino)phenylphosphine. The reaction with binuclear allyl compounds [Pd(μ-Cl)(allyl)]2 gives neutral [PdCl(allyl)P*] complexes. When allyl = 2-CH3-C3H4 (5), two isomers appeared in solution due to the R- or S-geometry around the palladium atom. The discrimination effect of the phosphines is small and the maximum isomeric ratio is observed for PPh(o-Ph2)(CH2SiPh3). The molecular structure determined by X-ray diffraction of two complexes with P* = PPh(o-Ph2)(i-Pr) and PPh(o-Ph2)(OMe) showed a very similar nonsymmetric coordination of the allyl moiety according to the greater trans influence of the phosphorus atom. When allyl = 1-C6H5-C3H4 (6), the NMR spectroscopy showed up to four isomers due to the R- or S-geometry around palladium and the Z- or E-disposition of P* and the phenyl substituent of the allyl moiety. The E-isomers are the major species in solution, unique with PPh(o-Ph2)(CH2SiPh3). The usual, well-defined dynamic exchanges by π−σ−π and pseudorotation of the allyl moiety have been observed. The codimerization reaction between styrene and ethylene has been tested using filtered CH2Cl2 solutions of [PdCl(2-CH3-C3H4)P*] (5) complexes and AgBF4 as catalytic precursors. Moderate activity (TOF < 225 h-1 at 25 °C) and good selectivities to 3-Ph-1-butene (∼90% at 80% conversion) are obtained. The ee is moderate (<40% ee) and different from the discrimination effects observed in the solutions of neutral complexes [PdCl(ally)P*]. The reaction carried out with deuterated styrene shows the clean C−H addition to the vinyl double bond of stryrene and confirms the irreversible nature of the insertion of styrene in the palladium hydride intermediate. The hydrovinylation reaction using substituted styrene with a potentially secondary coordination atom occurs only when the substitution is in the phenyl ring and without significant improvements of the ee.